Removing RHEL6 CI leg as it is not support in .NET 5.

.vsts-ci.yml

@@ -130,13 +130,6 @@ stages:
               _RuntimeIdentifier: ''
               _BuildArchitecture: 'x64'
               _DropSuffix: ''
-            Build_Rhel_6_Debug_x64:
-              _BuildConfig: Debug
-              _DockerParameter: '--docker rhel.6'
-              _LinuxPortable: ''
-              _RuntimeIdentifier: '--runtime-id rhel.6-x64'
-              _BuildArchitecture: 'x64'
-              _DropSuffix: ''
             Build_Arm_Debug:
               _BuildConfig: Debug
               _DockerParameter: ''
@@ -187,13 +180,6 @@ stages:
               _RuntimeIdentifier: '--runtime-id linux-arm64'
               _BuildArchitecture: 'arm64'
               _DropSuffix: ''
-            Build_Rhel_6_Release_x64:
-              _BuildConfig: Release
-              _DockerParameter: '--docker rhel.6'
-              _LinuxPortable: ''
-              _RuntimeIdentifier: '--runtime-id rhel.6-x64'
-              _BuildArchitecture: 'x64'
-              _DropSuffix: ''
             Build_Linux_musl_Release_x64:
               _BuildConfig: Release
               _DockerParameter: '--docker alpine.3.6'

eng/docker/rhel.6/Dockerfile

@@ -1,34 +0,0 @@
-#
-# Copyright (c) .NET Foundation and contributors. All rights reserved.
-# Licensed under the MIT license. See LICENSE file in the project root for full license information.
-#
-
-# Dockerfile that creates a container suitable to build dotnet-cli
-FROM microsoft/dotnet-buildtools-prereqs:centos-6-376e1a3-20174311014331
-
-# yum doesn't work with the special curl version we have in the base docker image,
-# so we remove /usr/local/lib from the library path for this command
-RUN yum -q -y install sudo
-
-# Setup User to match Host User, and give superuser permissions
-ARG USER_ID=0
-RUN useradd -m code_executor -u ${USER_ID} -g root
-RUN echo 'code_executor ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
-
-# With the User Change, we need to change permssions on these directories
-RUN chmod -R a+rwx /usr/local
-RUN chmod -R a+rwx /home
-RUN chmod -R 755 /usr/bin/sudo
-
-# Set user to the one we just created
-USER ${USER_ID}
-
-# Set library path to make CURL and ICU libraries that are in /usr/local/lib visible
-ENV LD_LIBRARY_PATH /usr/local/lib
-
-# Set working directory
-ARG WORK_DIR
-WORKDIR ${WORK_DIR}
-
-# Set up Azure Artifacts credential provider
-RUN wget -qO- https://raw.githubusercontent.com/Microsoft/artifacts-credprovider/master/helpers/installcredprovider.sh | bash